The invention relates to electrochemical sensors, biomedical testing, and blood analysis.
Electrochemical assays for determining the concentration of enzymes or their substrates in complex liquid mixtures have been developed. For example, electrochemical sensor strips have been developed for the detection of blood glucose levels. Electrochemical sensor strips generally include an electrochemical cell in which there is a working electrode and a reference electrode. The potential of the working electrode typically is kept at a constant value relative to that of the reference electrode.
Electrochemical sensor strips are also used in the chemical industry and food industry, to analyze complex mixtures. Electrochemical sensors are useful in biomedical research, where they can function as invasive probes, and for external testing (i.e., testing of blood obtained by a needle and syringe, or a lance).
Typical electrochemical sensors for blood analysis measure the amount of analyte in a blood sample by using a working electrode coated with a layer containing an enzyme and a redox mediator and a reference electrode. When the electrodes contact a liquid sample containing a species for which the enzyme is catalytically active, the redox mediator transfers electrons in the catalyzed reaction. When a voltage is applied across the electrodes, a response current results from the reduction or oxidation of the redox mediator at the electrodes. The response current is proportional to the concentration of the substrate. Some sensors include a dummy electrode coated with a layer containing the redox mediator but lacking the enzyme. The response current at the dummy electrode represents a background response of the electrode in contact with the sample. A corrected response is derived by subtracting the response of the dummy electrode from the response of the working electrode. This dummy subtraction process substantially eliminates background interferences, thereby improving the signal-to-noise ratio in the electrode system.
The invention features an electrode for use in an electrochemical sensor for measuring an analyte in a sample. The electrode includes a thin working layer. The thin working layer can be from 2 to 10 microns thick, and preferably is from 4 to 8 microns thick. Preferably, the thin working layer includes an enzyme and a redox mediator. Preferably, it also includes a binder, a film former, and a filler. In an electrode for measuring glucose, the enzyme uses glucose as a substrate, and preferably the enzyme is glucose oxidase or glucose dehydrogenase. Preferably, the thin working layer includes a redox mediator such as ferrocene, a ferrocene derivative, ferricyanide, or an osmium complex. The thin working layer of the electrode can be a printed layer, for example, a screen printed layer.
The invention also features an electrode strip for use in an electrochemical sensor for measuring an analyte in a sample. The electrode strip includes an electrode, which includes a thin working layer. The thin working layer can have a thickness of 2 to 10 microns. Preferably, the thickness is 4 to 8 microns. The thin working layer preferably includes an enzyme and a redox mediator. Preferably, it also includes a binder, a film former, and a filler. In an electrode strip for measuring glucose, the enzyme uses glucose as a substrate, and preferably the enzyme is glucose oxidase or glucose dehydrogenase. Preferably, the thin working layer includes a redox mediator such as ferrocene, a ferrocene derivative, ferricyanide, or an osmium complex. The thin working layer of the electrode can be a printed layer, for example, a screen printed layer. The electrode arrangement in the electrode strip can include a working electrode, a dummy electrode, and a reference electrode. Preferably, the reference electrode is downstream of the working electrode, relative to sample flow. The electrode strip can also include a hydrophilic mesh layer overlaying a sample loading area and the electrode arrangement. In addition, the electrode strip can include a cover layer defining an upper boundary of a cell volume encompassing the electrode arrangement, and an aperture in the cover layer, above the sample loading area.